In a fluidized bed power plant, the depth of the fluidized bed is controlled in dependence on the power used. It is necessary to maintain a certain temperature of the bed (typically of the order of magnitude of 850.degree. C.), to obtain a good degree of combustion and the highest possible absorption of sulfur. Part of the power is taken out through a nest of tubes which cool the bed and heat water or generate steam. At full load operation, the entire nest of tubes is submerged within the bed. The cooling capacity of this nest of tubes has to be adapted to the used power in order to maintain the proper bed temperature. When the power output decreases, the fuel supply and the power development in the bed will decrease but the cooling capacity of the nest of tubes will remain unchanged, which leads to an undesired reduction of the bed temperature. The cooling capacity of the nest of tubes can be reduced by reducing the depth of the bed so that part of the nest of tubes will then be located above the top surface of the bed. In the case of a load change, the cooling capacity is controlled by varying the bed depth so that the tubes will to a certain extent be located above the top surface of the bed so as not to unduly cool the bed. In the case of capacity control, the fuel and the air quantity must be controlled in dependence on the power taken out from the plant, while at the same time controlling the bed depth so that the cooling capacity of the nest of tubes is adapted to the energy supply, thus maintaining the correct bed temperature. In the case of a plant operating at a steady power level, the depth of the bed is maintained constant.
Sulfur-absorbing bed material such as limestone or dolomite must be supplied in an amount related to the sulfur content of the supplied fuel. In order to prevent the bed depth from rising, consumed bed material, such as slag and ashes, must be removed. Fine-grained material, such as finer fractions of ashes and well comminuted bed material, leave the bed with the combustion gases, and separated therefrom in a cleaning plant for subsequent removal. To maintain the bed depth constant or to reduce the bed depth, other bed material is removed through a feeding-out device at the bottom of the combustion chamber containing the fluidized bed. Such material, after crushing which exposes unconsumed sulfur-absorbent material, can be returned to the combustion chamber, as disclosed in Brannstrom et al's U.S. Pat. No. 4,421,036 (assigned to the assignee of this application).